Thermal emission sources have the advantage that an emission can be obtained by merely imparting heat to an object. For example, in a gas sensor for analyzing the components in an exhaust gas from an engine, the thermal emission source can be suitably used as an emission source for converting waste heat produced by the engine into infrared radiation for the sensing. The application areas of the thermal emission source are not limited to gas sensors; it can also be used as the emission source in various sensors which utilize infrared radiation, such as a sensor for detecting an intrusion of a person, automobile, animal or the like.
Electromagnetic waves emitted from an object which has been given heat have a spectrum which covers a wide range of wavelengths. For example, electromagnetic waves generated by heating an object to a temperature of tens to hundreds of degrees Celsius have a wavelength range of a few μm to several tens of μm. However, since the aforementioned infrared sensor normally utilizes only a specific wavelength of infrared radiation, using such a thermal emission source causes the measurement target to be irradiated with unnecessary wavelengths of infrared radiation other than the specific wavelength, which produces adverse effects, such as the heating of the measurement target. In the case of generating the thermal emission by supplying electric energy, using an emission source which generates a broadband emission causes the problem of an increase in the power consumption.
To solve such problems, a thermal emission source is proposed in Non Patent Literature 1, which includes a slab having a quantum well structure (which will be described later) and modified refractive index areas cyclically arranged in the slab, with the refractive index of those areas differing from that of the slab. A quantum well structure is the structure of an object in which an energy potential in the form of a well (“quantum well”) is created by stacking a plurality of kinds of semiconductor layers with a thickness of approximately a few nm to a dozen nm, with each layer having a different magnitude of energy band gap. The cyclic arrangement of the modified refractive index areas in the slab creates a cyclic distribution of the refractive index. Such an object consisting of a slab in which a cyclic distribution of refractive index is formed is called the “two-dimensional photonic crystal”. It is commonly known that a two-dimensional photonic crystal can act as a resonator for light having a specific wavelength corresponding to the lattice constant of the refractive index distribution by enabling the formation of standing waves of that specific wavelength of light.
In the thermal emission source disclosed in Non Patent Literature 1, when the supply of heat from the heat source is initiated, an energy transition occurs between the energy bands formed in the quantum well (for distinction from the energy bands of the semiconductor, those bands are called “subbands”), whereby a range of wavelengths of light within a specific wavelength band are generated. Among the wavelengths of light belonging to this wavelength band, the light having a specific wavelength determined by the lattice constant of the photonic crystal resonates with the photonic crystal. In this manner, the thermal emission source of Non Patent Literature 1 can generate light having a wavelength spectrum which has a narrow peak at that specific wavelength.